This invention comprises improvements in the field of particle encapsulation, especially relating to an improved method for the microencapsulation of pigments. The present invention relates directly to improvements over the particle encapsulation system disclosed in U.S. Pat. No. 3,946,134.
The use of metal-oxygen-phosphorus bonded polymers, especially aluminum-oxygen-phosphorus bonded polymers, to form protective coatings is well known. Network structures with the Al--O--P configuration have been prepared (H. H. Greger, U.S. Pat. No. 2,460,344) and used as insulating coatings. U.S. Pat. No. 3,538,136 discloses more sophisticated polymers based on the Al--O--P structural unit where the phosphorus, commonly in the form of phosphate, has been replaced with a phosphonate or phosphinate analog. More recently U.S. Pat. No. 3,955,017 claims transparent vitreous metal phosphate coatings on organic polymers derived from a metal phosphate complex. U.S. Pat. No. 3,946,134 uses an AlPO.sub.4 complex to form microcoatings or capsules around particles.
This last system (U.S. Pat. No. 3,946,134) for the encapsulation of particles contrasts sharply with the then existing art in that it is applicable to both organic and inorganic particles, and produces a continuous, dense, protective capsule around each particle that provides one or more of at least ten recited advantages including improvements in one or more such characteristics as chemical stability, thermal stability, and light stability. The encapsulation is achieved by the slow homogeneous nucleation of a boric acid aluminum phosphate complex at the surface of the particles in an aqueous dispersion. The capsules so produced are between 15 and 200 A thick as determined by electron microscopy and are strongly bonded to the substrate particle by two further steps consisting of neutralization of the dispersion and subsequent dehydration of the encapsulated particles.
While this is a significant advance over prior art processes, certain deficiencies still remain in the process, resulting in a less than optimum encapsulation. In particular, the deposition of the coating is neither as efficient nor as controllable as desired. Further, the capsules in their final form are not as dense, impervious, and adherent as is possible. Additionally, the required dehydration step is a serious disadvantage if the particles are to be used in a wet state or processed further. Heretofore it was not readily apparent how these deficiencies could be overcome.